The present invention is directed to the manufacture of printed circuit boards using a plating, etching or permanent resist material. More specifically, the present invention is directed to removal of used resist subsequent to plating and/or etching operations.
In the manufacture of printed circuit boards, a temporary polymeric resist image is formed on a substrate, e.g., a copper clad fiberglass epoxy laminate which exhibits uncovered surface areas not protected by the resist image that are modified by either removal with an etchant or by depositing an additional material through plating. After the surface areas are modified, the resist image is typically removed before subsequent steps are preformed to produce a printed circuit board. Removal of the resist image is facilitated by use of liquids and vapors which are typically organic solvents, aqueous alkaline solutions or combinations thereof. Such liquids are chosen so that the substrate material and the overlying circuit lines are left substantially unaffected by the liquid treatment and are left substantially residue free. Circuit line density on printed circuit boards may be greater than 0.010 inch wide circuit lines and spaces or as small as about 0.002 inch wide circuit lines and spaces. Frequently circuit density will vary substantially from one area of a circuit board to another. As circuit density becomes greater, resist removal becomes more difficult requiring longer chemical exposure time and additional controlled conditions to satisfactorily produce fine line circuitry. Since the liquids and vapors used to remove the resist frequently are vigorous organic solvents and/or caustic solutions, added safety measures are required to carry out this operation. Safe environmental disposal of spent waste liquids add further concern. The production of printed circuit boards using a variety of resists is described in "Printed Circuits Handbook" edited by Clyde F. Coombs, Second Edition, published by McGraw-Hill, Inc., 1979. In particular, pages 8-9 through 8-11 discuss resist removal in detail. The present invention eliminates the need of liquid/vapors for resist removal. It renders a much simpler, less costly and more environmentally safe method of resist removal.
Accordingly, sand blasting and grit blasting with inorganic media has been used to abrasively remove paint and other adherent materials for cleaning underlying surfaces. Sand and grit blasting, for delicate substrates such as airplane skins and composites, is too vigorous for practical use. Therefore, organic media grit has been used instead for periodic paint removal. U.S. Pat. No. 4,731,125 discloses a soft media blast paint removal system which utilizes granular plastic media for blast cleaning paint from composite surfaces without rendering damage to the surface. The media disclosed in the above-referenced patent have a Moh hardness in the range of 2.5 to 3.5 and a U.S. standard sieve sizes ranging from 12-16 to 60-80. In this system the plastic media is propelled to give a media flow of about 40 p.s.i. and the media flow path will be one in which the angle and direction of the flow produces highly efficient removal of adherent material from the surface without damaging the composite. Typically, the media flow is angled away from a perpendicular direction with respect to the target surface so that the leading edge of the coating is exposed to the media flow. Likewise, U.S. Pat. No. 4,759,774 discloses a filled polymeric blast media for similar use.
Polymeric resin abrasives have been used in blasting operations to remove flash from molded encapsulants of electronic circuit components, such as: resistor, capacitor, semiconductor and the like. U.S. Pat. No. 4,548,617 discloses synthetic resin, abrasive particles each having a plurality of cutting edges and cracks for cleavage upon impact. The resin particles are used as blast media for flash removal from plastic encapsulated semiconductor packages on lead frames without damaging the molded product. The patented invention discloses the hardness of the resin media which is equal or close to that of the encapsulant is optimal, and the average particle size is preferably within the range of 0.05 to 2.0 mm.
A need exists in the printed circuit industry to simplify and shorten the resist removal process without damaging the delicate printed circuit lines or the underlying substrate material. The demand extends to a process which yields an environmentally safe waste, one without caustic liquids intermingled with spent resist; waste which could be disposed of in an ordinary landfill.